1. Technical Field
The present disclosure relates to medical devices, in particular, to an implantable lead, a medical device using the same and a method for making the same.
2. Description of Related Art
In recent years, implantable medical devices (IMD), such as heart pacemakers, defibrillators are widely and increasingly applied in treatments of diseases. Currently, there are more than 5 million implanted patients in the world. Furthermore, deep brain stimulator (DBS) has demonstrated its remarkable success in treating movement disorders such as Parkinson's disease, essential tremor and dystonia. Also, spinal cord stimulators are widely used in the treatment of pain. Other IMD such as vagus nerve stimulator, sacral nerve stimulator, stomach stimulator, bladder stimulator have been developed and put into clinical applications.
Magnetic resonance imaging (MRI) is widely used in modern medical diagnosis because of MRI is non-radioactive, has high-resolution, capable of soft tissue imaging, and development brain function. It is estimated that the global annual MRI examination is more than 60 million times and will rise continuously. However, the risk for patients having the IMD implants, such as heart pacemaker, defibrillator, or nerve stimulator, is significant mainly due to the radio frequency (RF) heating produced during the MRI examination.
The MRI utilizes three electromagnetic fields to function. The first one is a static magnetic field B0 used to provide a uniform magnetic field environment. The second one is a gradient magnetic field used to generate spatial position information. The third one is a RF magnetic field used to excite an MR signals. The RF magnetic field has a high-power and is high-frequency time-varying magnetic field. The frequency f of the RF magnetic field is determined by the Larmor formula f=γB0, wherein γ is gyromagnetic ratio with a value of 42.5 Hz/T. According to Faraday's law of electromagnetic induction, changes of the RF magnetic field will induce electric fields in biological tissues. When a slender metal is implanted in a biological tissue, such as the heart pacemaker lead or the DBS lead, the slender metal will receive the RF signal like an antenna and cause an induced electric field aggregate at the tip of the slender metal to produce a severe ohmic heat which is called RF heating.
In a phantom study, a temperature rise up to 63° C. is observed and a temperature rise up to 20° C. is observed in animals when 1.5 T MRI is used to scan. Such a high temperature may cause serious harm to patients. For example, the United State Food and Drug Administration (FDA) had reported that the implantation of DBS in Parkinson patients for MRI examination leads to coma and permanent disability cases. The above risk causes about 200 thousand patients refusing the MRI examinations. However, it is reported that about 50%-70% of patients with the implants need the MRI examinations in the life cycles of the IMD.
Thus, it is important to provide a safe IMD in the MRI environment, especially in the case of heart pacemakers, defibrillators, or nerve stimulators, without RF heating resulting a severe temperature rise at the lead during the MRI examination. Since the RF heating is mainly produced at the lead, it is very useful to provide improve leads which are safe to use in the MRI environment.
Metal shielding layer is widely used in the protection of the cable by shielding the electromagnetic radiation. However, metal shielding layer structure cannot reduce RF heating effectively because of the thick insulating layer outside of the cable. Furthermore, metals with good biocompatibility are rare and expensive. Properties of a potential suitable metal are also difficult to maintain after the metal is made into thin shielding layer. In U.S. patent Publication No. U.S. 2008195187A1 published on Aug. 14, 2008, entitled “DISCONTINUOUS CONDUCTIVE FILLER POLYMER-MATRIX COMPOSITES FOR ELECTROMAGNETIC SHIELDING”, filed on Feb. 14, 2007, a polymer composite is applied to the shielding layer. However, the shielding effect is relatively poor, and the shield layer needs to be made very thick to take advantage of the properties of the polymer composite, while makes the shielding layer become unsuitable for the IMD.
What is needed, therefore, is to provide an implantable lead, a medical device using the same and a method for making the same which can overcome the shortcomings described above.